There are optical discs such as a CD including a read only CD and a writable CD-R disc which is used as a ROM. Further, as a writable optical disc having a high recording density, a write once disc and an erasable disc are provided. Information is recorded on the disc and reproduced with a laser beam. These discs are different from the CD in the material of the recording surface.
For example, the write once disc has a tellurium or bismuth recording surface on which the lasers burn pits for recording. In another type of the write once disks, the lasers are focused on a recording surface coated with a selenium antimony (Sb.sub.2 Se.sub.3) thin film, or an oxide tellurium (TeOx) thin film, or a thin film of organic pigment, changing the reflectivity of the light.
The erasable disc uses as the recording surface, an amorphous alloy made of rare earth metals such as gallium, terbium, and others. In a magneto-optical recording method, the recording surface of the disc is initially magnetized to form a magnetic field in a direction perpendicular to the surface. The laser heats a predetermined area of the disc to elevate the temperature above Curie temperature, which is about 150.degree. C., thereby reversing the direction of the magnetic field. To read the recorded information, the laser is irradiated on the recording surface so that polarized wave front slightly rotates as a result of the Kerr effect. Thus only the polarized wave deflected by the rotation is read by a photodetector, thereby enabling to read the information.
In the write once disc and the erasable disc, an interchange sector or a TOC area is provided for recording information representing a table of contents (TOC) of the recorded information. When reproducing the information on the disc, a pickup head reads the interchange sector or the TOC area to know the position of an unrecorded area. Thus, useless reading of the unrecorded area on the disc can be avoided.
However, in such a disc, since it is necessary to provide a space for the interchange sector or the TOC area, the space for recording the information is reduced to reduce the amount of recording data, which means reduction of the recording density of the disc.
Furthermore, if there is an accidental omission in the recorded information, the pickup head may blindly run without controlling.